Extensometer



May 16, 1967 1. P. DE NlcoLA 3,319,338

EXTENSOMETER Filed May 10, 1965 United States Patent O 3,319,338 EXTENSOMETER Joseph l. De Nicola, Quincy, Mass., assigner to lnstron Corporation, Canton, Mass., a corporation of Massachusetts Filed May 10, 1965, Ser. No. 454,499 9 Claims. (Cl. 323-148) This invention relates to extensometers and more pla-rticularly to such devices in which strain sensors are mounted on a connector portion between a pair of specimen-engaging legs.

It is la primary object of the invention to provide such an extensometer, in which movement of the portions of the specimen engaged `by the legs co-rrespondingly moves the legs, in which extraneous forces on the legs are minimized and greatly increased linearity results. It is a further object of the invention to provide such an extensometer which is accurate and lightweight, and which is of relatively low cost, simple design, low hysteresis characteristics and sufficiently rugged construction to allow its use in stressing a sample to rupture. In particular, it is an object of the invention to provide a pair of clamps cooperating with the legs to hold a specimen thereagainst, the clamps being rotatably mounted relative to the legs, about axes coincident `)Vith the centerlines of contact of the legs with the specimen. A further important object, in the preferred embodiment, is to provide a spacer arrangement in which a leg may -be held against a spacer while loading the specimen lbetween the clamps and extensometer, to obtain a predetermined distance between the places (bench marks) where the specimen is clamped, and in which the leg moves slightly away from the spacer when pressure on it is released, to provide an initial setting without stress on the legs, yet without affecting the distance between the bench marks. Further objects of preferred embodiments are to provide a housing for the connector portion and the circuitry, mounted on one leg; and to bring off iwi-res from the strain sensors to affix them to the same leg, within the housing and adjacent where the housing is mounted.

The invention features a pair of specimen-engaging legs, with .a pair of clamps cooperating with corresponding ends of the legs to hold them against a specimen, the other ends of the legs `being joined by a connecting portion bearing a strain-sensing unit, the clamps being rotatable relative to .the legs about an axis colinear with the specilinen-engaging centerlines of the legs. In the most preferred embodiment, the specimen-engaging portions of the legs are knife-edges, and the legs also each carry a pair of oppositely directed colinear knife edges about which clamps are rotatable; the housing and wires are dealt with as above mentioned, providing security of function without undue imposition of extraneous force; and stress-free initial setting is provided as above set forth.

Other objects, features, and `advantages will appear from the following description of `a preferred embodiment of the invention, taken together with the attached drawings thereof, in which:

' FIG. 1 is a partially broken away View of said preferred embodiment with the protective cover removed;

FIG. 2 is an enlarged perspective view of the upper leg and clamp assembly shown in FIG. l;

FIG. 3 is an enlarged perspective view of the connector portion shown in FIG. l;

FIG. 4 is a sectional view taken at 4*4 of FIG. l.

Referring now more particularly to the drawings, there is shown in FIG. l an extensometer with legs 2 and 4 and connector portion 6. Each of the legs 2 and 4 is directed inwardly at its specimen-engaging end so that these portions 8 and 10 respectively, are approximately parallel. Secured to the ends of the legs are hardened steel mountice ings 12 and 14 which engage the specimen 16. Mounting 12 contains three knife edges 18, 20 and 22, which are colinear, as shown more clearly in FIGS. 2 and 4. Knife edge 20 engages the specimen 16, while knife edges 18 and 22 engage clamp 24 at corners 26 and 28 respectively.

`Clamp 24 ybasically comprises an approximately U shaped element 30 which engages the mounting 12 and a biasing unit 32. Each arm of the U shaped element 30 contains recesses 34 and 36 which contain the corners or bearing surfaces 26 and 28 which engage the knife edges 18 and 22. The angles at the corners 26 and 28 are sulficiently large to accept the shoulders 38 and 40, so that in use contact is only at the knife edges 18 and 22, allowing clearances 42 and 44 for any rotational movement of the clamp 12 about the knife edges 18 and 22.

Biasing unit 32, shown in more detail in FIG. 4, contains a nylon plug 46, a plug guide 48 and a spring 50 which cooperate to exert a pressure on the specimen 16, forcing it against knife edge 20. Plug 46 passes through aperture 5S in member 30. Plug guide 48 is press-fitted Vin plug 46 in seat 52, and retaining ring 54 mounted in groove 56 prevents spring 50 from forcing the plug completely out of its channel 60. Plug 46 has angular groove 62 in its .specimen contacting end, permitting the seating of round or irregularly shaped specimens and also lessening the friction between the plug and the specimen. The specimen engaging ends yare defined also by arcs of a circle, to minimize areas of contact with flat specimens.

Clamp 64 and mounting 14 are constructed and cooperate in the same manne-r Ias mounting 12 and clamp 24.

Spacer 66 is `disposed between the approximately parallel portions 8 and 10 of legs 2 and 4. The spacer 66 functions to limit movement of the legs. The bottom stop 68 and bottom stop rod 70, which passes through aperture 72 in leg 4, in conjunction with the top stop 74 and top stop rod 76, which passes through aperture 78 in leg 2, cooperate to maintain alignment of the legs and prevent their being separated more than a predetermined amount. When the extensometer is arranged to begin testing, there will be a gap 80 between leg 2 and spacer 66. I

Leg 4, connection portion 6, and leg 2, formed of one piece in this preferred embodiment, constitute the element 84 of the extensometer. The element 84 is arranged in the protective base 82 so that the only contact between them is along the length 86 of leg 2, where it is secured to the base in channel 88. The protective cover 142 has no contact with element 84. The protective cover is fastened to the protective base by pins, not shown, which are secured in recesses 90 and 92.

The sensors 94 and 98 are bonded t-o surface 102 of connection portion 6 and connected electrically to posts 106, 108 and 110, 112, respectively. Portion 6 of the element 84 is approximately straight to permit the bonding surfaces 102 and 104 to be manufactured within close tolerances in both dimensions of both, as well as in the dimension between them. Leads 114, 116, 118 and 120 are connected to their respective binding posts 106, 108, and 112 and arranged with strain relief loops 122, 124, 126 and 128. These leads are then `disposed in close, but not constrictive, relation to element 84 until they are directed away from the element approximately in the area where the element enters channel 88. These leads are bonded to the element 84 at the point where they begin their departure from it.

In a similar manner sensors 96 and 100 are bonded to surface 104, connected to posts, and have leads connected to them. These lea-ds are arranged and disposed in a like manner so that all leads leave the element at about the same place, at which the element is little sensitive to their influence.

The leads 114, 116, 113 and 12@ and their counterparts from surface 194 are connected to posts 136, mounted in insulating plate 132, to form a balanced bridge circuit of the sort well known in the art and including balancing resistor 134. The bridge circuit is connected to amplifying and read out means by four leads 136, which are led through a grommet 13S and form a `cable containing a shielding means 140.

In operation, the extensometer may be gripped with a finger lof one hand on the outside of upper leg 2 as close as possible to portion 8 and t-he thumb on the outside of lower leg 4 as close as possible to portion 10. With the specimen 16 mounted in the grips of a suitable testing machine, the extensometer is placed against the specimen so that knife edge 20 and its counterpart on leg 4 are in contact with the specimen. The top clamp 24 is positioned against the opposite side of the specimen; the force required is only enough to overcome the spring 50 bias and allow the clamp to clear the knife edges 18 and 22. After Ipositioning the knife edges in corners 26 and 2S, the pressure may be released; the spring biased plug 46 will maintain the correct seating of the clamp and the specimen. Similarly lower clamp 64 is positioned on the lower mounting 14. Releasing the grip on the legs will cause the legs to rotate about knife edge 20, and its counterpart on mounting 14, as fingerimposed `small distortions in the legs are released to provide a small gap 80 without any change in the distance between the bench marks. This gap insures that the extensometer will have no strain on it prior to testing but will always set up on the proper bench marks for which the device was designed and calibrated.

Other embodiments will occur to those skilled in the art and are within the following claims.

What is claimed is:

1. An extensometer for measuring strain in a specimen comprising:

an element with a pair of legs joined by a connector portion, each of said legs terminating away from said connector portion in a specimen-engaging surface,

a strain sensing unit mounted on said connector portion,

a clamp rotatably mounted on each of said legs about an axis defined by the said `specimen-engaging surface of the respective leg for holding said specimen thereagainst,

said axis and the centerlines of said specimen-engaging surfaces being colinear.

2. The extensometer of claim 1 in which said specimenengaging surfaces are knife edges and each leg carries a pair of oppositely directed knife edge bearings having surf-ace centerlines colinear with each other and with that of the specimen-engaging knife edge, said clamps being rotatable on said bearings.

3. The extensometer of claim 1 which includes a housing for said connector portion and at least a fraction of said legs, said housing being carried by one of said legs and Ibeing free of Contact with the other of said legs and with said connector portion.

4. The extensometer of claim 1 in which said clamps each comprise a handle,

.a pair of bearing arms extending in parallelism in a rst direction and away from said handle, each of said arms carrying a bearing surface facing generally toward said handle, the bearing surfaces being accessible through notches in said arms, each said notch extending in a direction generally transverse to said first direction,

.a plug mounted in said handle for movement relative to said bearing arms along said direction, said plug being limitedly movable away from said handle,

the centerlines of said bearing surfaces and said plug being coplanar.

5. The extensometer of claim 2 which includes a leg spacing member extending between said legs, said leg spacing member including a central portion between said legs and two end portions, the central portion being provided with opposed stop surfaces oriented to cooperate respectively with said legs to limit movement thereof toward each other and each end portion being provided with a stop surface to limit movement of one of said legs away from said central portion, said leg spacing member being mounted on said legs to permit free movement of each said leg between said central portion and the respective of said end portions, whereby a specimen may be introduced while said legs are held against the central portion stop surfaces and thereafter upon release of said legs, the legs resume a configuration unstressed by forces intermediately thereof by rotation about said centerlines and slight movement away from said central portion stop surfaces.

6. The extensometer of claim 3 in which electrical leads from said strain sensing unit are adhered to the leg carrying said housing where said leads are led from said element.

7. The extensometer of claim 4 in which said plug material is nylon, and carries a channel in its specimencontacting portion, said channel bounded by sides which are generally in parallelism with said channel centerline, each of said sides being t-ape-red to an edge which forms an arc of a circle.

8. The extensometer of claim 4 in which:

said specimen-engaging surfaces are knife edges and each leg carries a pair of oppositely directed knife edge bearings having surface centerlines colinear with each other and with that of the specimenengaging knife edge,

said clamps are rotatable on said bearings, and

said clamps each comprise a handle, a pair of bearing arms extending in parallelism in a first direction and away from said handle, each of said arms carrying a bearing surface facing generally toward said handle, said bearing surfaces being corners and accessible through notches in said arms, each said notch extending in a direction generally traverse t0 said first direction, and a plug mounted in said handle for movement relative to said bearing arms along said direction, said plug being limitedly movable away from said handle, the centerlines of said bearing surfaces and said plug being coplanar.

9. The extensometer of claim 8 which includes a spacer disposed between said legs, a rod fastened to each end of said spacer and passing through an a erture, in a related one of said legs, terminated and fastened in a stop, said `spacer ends being fulcrums about which said specimen-engaging surfaces rotate in response to a force applied to said legs between said connector portion and said spacer urging said legs toward said spacer, said spacer being separated from said legs upon release of said force.

References Cited by the Examiner UNITED STATES PATENTS 2,543,429 2/1951 Wood 33-148 2,611,966 9/1952 Rebman 33-148 2,666,262 1/1954 Ruge 33-148 2,767,476 10/1956 Strimel 33-148 2,768,477 10/1956 Strimel 33-148 2,814,883 12/1957 Strimel 33-147 3,158,939 12/1964 Brooks 33-148 LEONARD FORMAN, Primary Examiner.

W. D. MARTIN, Assistant Examiner. 

1. AN EXTENSOMETER FOR MEASURING STRAIN IN A SPECIMEN COMPRISING: AN ELEMENT WITH A PAIR OF LEGS JOINED BY A CONNECTOR PORTION, EACH OF SAID LEGS TERMINATING AWAY FROM SAID CONNECTOR PORTION IN A SPECIMEN-ENGAGING SURFACE, A STRAIN SENSING UNIT MOUNTED ON SAID CONNECTOR PORTION, A CLAMP ROTATABLY MOUNTED ON EACH OF SAID LEGS ABOUT AN AXIS DEFINED BY THE SAID SPECIMEN-ENGAGING SURFACE OF THE RESPECTIVE LEG FOR HOLDING SAID SPECIMEN THEREAGAINST, SAID AXIS AND THE CENTERLINES OF SAID SPECIMEN-ENGAGING SURFACES BEING COLINEAR. 